This invention relates to a three dimensional (3-D) seismic prospecting method wherein data is collected for a 3-D seismic areal array of sources and receivers and wherein such data is processed in a manner which optimizes certain seismic parameters which are discussed further below.
In 3-D seismic prospecting, an areal array of seismic sources and receivers are positioned over an area of the earth's surface and seismic data is collected in the form of seismic traces which are generated by the receivers in response to reflected acoustic waves. This is in contrast to two dimensional seismic prospecting wherein a line rather than an areal array of sources and receivers is utilized. In 3-D as well as in two dimensional seismic prospecting it is desirable to "stack" a number of traces (commonly called a common midpoint bin or gather) which correspond to a number of source-receiver pairs which share a common midpoint. As used herein, the term "source-receiver pair" refers to a source position and receiver position located on opposite sides of a common midpoint and spaced substantially equidistantly from the common midpoint. Stacking of seismic traces corresponding to such source-receiver pairs involves summing of the traces so as to enhance important reflection events in the traces and remove spurious noise which can obscure the reflection events. In other words, stacking enhances the signal to noise ratio.
Certain parameters which characterize a group of 3-D source-receiver pairs corresponding to a particular common midpoint include fold, offset and azimuth. Fold refers to the number of source-receiver pairs sharing a common midpoint for which traces are stacked. For example, if there are 16 source-receiver pairs for a particular stack, there are 16 folds. Offset is simply the distance between the source and receiver of a particular source-receiver pair. Azimuth is the angular orientation of the source-receiver pair. More precisely, the azimuth angle for a particular source-receiver pair is the angle defined between the line along which the source-receiver pair lies and a preselected direction such as true east or north.
In planning a 3-D seismic areal array according to conventional techniques, it is desirable to position the sources and receivers to optimize certain conditions with respect to fold, offset and azimuth.
With respect to fold, it is desirable to have an adequate number of folds for each common midpoint in order to give an acceptable signal to noise ratio in the resulting stacked trace. It is also desirable to have uniformity of fold among a maximum number of common midpoints for a particular areal array. This results in a uniform signal to noise ratio for the various stacked traces. With such a uniform signal to noise ratio among stacked traces, any variation of amplitude from trace to trace will be related to the strength of reflection events and not the difference in the number of traces being summed. This makes seismic interpretation easier and more accurate.
With respect to offset, it is desirable to have a maximum variation of offsets for the source-receiver pairs corresponding to a particular common midpoint. The different offset values are utilized to derive an average stacking velocity for the traces being stacked. Such a stacking velocity is used to correct for normal moveout among the traces. Maximizing the distribution of offset values serves to enhance the accuracy of the derived stacking velocity and thus also the accuracy of the resulting normal movement correction.
With respect to azimuth, it desirable to have a maximum variation in azimuth angles among the source-receiver pairs corresponding to a particular common midpoint. By having many different azimuth angles, the accuracy of 3-D statics solutions is enhanced. Statics are corrections applied to seismic data to correct for low velocities (weathering velocities) of seismic waves encountered in unconsolidated sediments near the earth's surface.
Planning the positioning of sources and receivers in a 3-D seismic areal array to optimize the various parameter conditions discussed above is typically done by trial and error placement of sources and receivers until the desired optimization of conditions is obtained. Such a procedure is extremely time consuming. Depending on the size of the areal array, such a procedure can take from about a week to several weeks to carry out. This translates to a high expense and an adverse effect on the efficiency of a particular seismic prospecting project.